We investigate the dispersion properties of leaky-mode resonance elements with emphasis on slow-light applications. Using particle swarm optimization, we design three exemplary bandpass leaky-mode devices. A single-layer silicon-on-insulator leaky-mode element shows a time-delay peak of ~8 ps at the resonance wavelength. A double membrane element exhibits an average delay of ~6 ps over ~0.75 nm spectral bandwidth with a relatively flat dispersion response. By cascading five double-membrane elements, we achieve an accumulative delay of ~30 ps with a very flat dispersion response over ~0.5 nm bandwidth. Thus, we show that delay elements based on leaky-mode resonance, by proper design, exhibit large amount of delay yet very flat dispersion over appreciable spectral bandwidths, making them potential candidates for optical buffers, delay lines, and switches.